Heavy wall seamless steel pipe for line pipe and a manufacturing method thereof

ABSTRACT

A heavy wall seamless steel pipe for line pipe with a high strength and increased toughness, which has a chemical composition, by mass %, that consists of C: 0.03 to 0.08%, Si: not more than 0.25%, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to 0.008%, and 0.0002 to 0.005%, in total, of at least one selected from Ca, Mg and REM, and the balance Fe and impurities, optionally including V: 0 to 0.08%, Nb: 0 to 0.05% or Cu: 0 to 1.0%, and that P and S among impurities are not more than 0.05% and not more than 0.005% respectively. It may contain 0.0003 to 0.01% of boron. A manufacturing method thereof is characterized by cooling rate, heating condition for piercing, and heat treating after pipe making.

TECHNICAL FIELD

The present invention relates to a heavy wall seamless steel pipe forline pipe excellent in strength, toughness and weldability, and amanufacturing method thereof. The heavy wall seamless steel pipe means aseamless steel pipe having a wall thickness of 25 mm or more. Theseamless steel pipe of the present invention is a high-strength,high-toughness heavy wall seamless steel pipe for line pipe having astrength of not less than X70 regulated in API (American PetroleumInstitute) Standard, that is, a strength of X70 (yield strength of 482MPa or more), X80 (yield strength of 551 MPa or more), X90 (yieldstrength of 620 MPa or more), X100 (yield strength of 689 MPa or more),and X120 (yield strength of 827 MPa or more), which is particularlysuitably used for submarine flow lines.

BACKGROUND ART

In recent years, petroleum and gas resources of oil fields located onland or in shallow sea areas are running dry, and development ofoffshore oil fields in deep sea areas has been increasingly activated.In deep-sea oil fields, there is a need for the transportation of crudeoil or gas from a pit of an oil well or gas well set on the sea bottomto a platform on the ocean by the use of a flow line or riser.

An inner part of a pipe constituting the flow line laid in the deep seasuffers a high internal fluid pressure in addition to a deep stratumpressure, and is also subjected to repeated strains by ocean waves andinfluenced by the sea water pressure of the deep sea at the time ofshutdown. Therefore, a heavy wall steel pipe with high strength andtoughness is desired as the pipe used for this purpose.

Such a seamless steel pipe with high strength and toughness has beenmanufactured by piercing a billet heated to high temperature by apiercing mill, shaping into a pipe shape of product by rolling anddrawing, and then performing heat treatment. In recent years, however,simplification of the manufacturing process by applying an in-line heattreatment has been examined from the viewpoint of energy and processsaving. Particularly, paying attention to effective use of the heatpossessed by the material after hot-worked, a process for performingquenching without once cooling beforehand to room temperature has beenintroduced. According to this method, substantial energy saving andincreased efficiency of the manufacturing process can be attained,enabling significant reduction in manufacturing cost.

A steel pipe manufactured in the in-line heat treatment process ofperforming quenching directly after finish rolling has not beensubjected to transformation and reverse transformation since, unlike inthe past, it is not reheated after once cooling to room temperature androlling. Therefore, the grains are apt to be coarsened, and it is noteasy to ensure the toughness and corrosion resistance. Some techniqueshave been proposed in order to make fine grains of a finish-rolled steelpipe and ensure the toughness or corrosion resistance even if the grainsare not so fine.

For example, the following Patent Document 1 (Japanese Patent UnexaminedPublication 2001-240913) discloses a technique for making fine grains byadjusting the leading time to let it into the reheating furnace afterfinish rolling. The following Patent Document 2 (Japanese PatentUnexamined Publication 2000-104117) discloses a technique for adjustingthe chemical composition, particularly, the contents of Ti and S toprovide a satisfactory performance even with a relatively large grainsize.

However, the technique disclosed in Patent Document 1 cannot respond tomanufacture of a heavy wall steel pipe with high strength for offshoreoil fields in depth, which has been increasingly demanded in recentyears. For example, the heavy wall steel pipe requires a high finishrolling temperature, and it takes an excessive time to ensure anintended reheating furnace temperature, and seriously reduces theproduction efficiency. The method described in Patent Document 2 is alsohardly applicable to heavy wall materials. Since the cooling rate in thein-line heat treatment is reduced in the case of heavy wall materials,the toughness is deteriorated even if steel of the composition disclosedin Patent Document 2 is applied.

[Patent Document 1] Japanese Patent Unexamined Publication 2001-240913.

[Patent Document 2] Japanese Patent Unexamined Publication 2000-104117.

DISCLOSURE OF THE INVENTION Problems to be Solved by the Invention

In order to solve the above-mentioned problems, it is the objective ofthe present invention to provide a seamless steel pipe for line pipehaving high strength and stable toughness, particularly, in a steel pipewith a heavy wall and also a manufacturing method thereof.

Means for Solving the Problems

1. Fundamental Examination and Findings

Factors governing the toughness of a heavy wall seamless steel pipe wereanalyzed first. As a result, the following information was found.

(1) Cooling conditions in and after solidification of molten steelgreatly influence the toughness. Since a lower cooling rate causesreduction in toughness, the cooling must be accomplished at not lessthan a specific cooling rate.

(2) A blooming process for heating an ingot to a high-temperature rangefor hot working does not have a good influence on the toughness.

(3) The above-mentioned reduction in toughness is caused by theprecipitation form of Ti carbonitride due to the cooling rates in andafter solidification. In order to prevent this reduction in toughness,it is important to finely precipitate the Ti carbonitride.

(4) Precipitation strengthening deteriorates the balance betweenstrength and toughness in the case of in-line heat treatment materials.Although it is disadvantageous for obtaining a high strength, it isdesirable to utilize the transformation strengthening and thesolid-solution strengthening, without utilizing the precipitationstrengthening, in order to obtain a high toughness.

(5) It is necessary to prevent from the generation of a retainedaustenite and a low-temperature transformed martensite in order toobtain a homogenous metal microstructure.

(6) Regarding the chemical composition, it is desirable to reduce thecontents of Si, P and S, and to control the contents of Nb and V so asnot to exceed the specific upper limits, and also to include a properquantity of Ti, in addition to a proper quantity of at least oneelements selected from Ca, Mg and REM. Accordingly, the toughness ofheavy wall materials will be significantly improved.

(7) The findings described in (1) to (6) above were obtained on theassumption of the in-line heat treatment. However, if it is applied to asteel pipe subjected to an off-line heat treatment, an increasedtoughness can be obtained. Therefore, the above-mentioned findings canalso be used to manufacture a high-strength material by an off-line heattreatment.

2. Basic Test and Result

Since an in-line heat treatment does not have a fine-grain makingprocess by “transformation-reverse transformation” unlike an off-lineheat treatment, a fine-graining itself at the end of rolling is requiredto ensure the toughness.

It is generally said that although the as-solidified grains are coarse,the grains become fine by reheating in order to perform blooming.Therefore, optimization of the blooming process in the in-line heattreatment materials was examined under laboratory experiments. As aresult, it was found that, when the blooming is not executed, the grainstend to be fine in the in-line heat-treatment material, improving thetoughness. Namely, it was found that the conventional general knowledgeis not always correct.

In order to understand this unexpected result, a simulation test wasfurther executed under laboratory experiments. In the process includingthe blooming process, a cast ingot was heated to 1250° C. and hot workedto form a block, which was then further heated to 1250° C. to performhot rolling and water cooling, whereby the piercing process and thein-line heat treatment process were simulated.

In the process without including the blooming process, a block of thesame size as the block formed by the above hot working was cut from thecast ingot by machining, and this block was heated to 1250° C. toperform hot rolling and water cooling, whereby the piercing process andthe in-line heat treatment process were simulated.

As the results of the two simulation tests, the grain size not subjectedto blooming was overwhelmingly fine and the toughness was improved.

However, the similar trial to the two simulation tests executed underactual equipments, not under laboratory experiments, resulted in thefact that the grains not subjected to blooming were not finer thanexpected.

Therefore, the inventors examined why the grain size not subjected toblooming differed extremely from the one subjected to blooming about thetwo simulation tests under laboratory experiments.

As a result, they found that most of added Ti precipitated as Ticarbonitride in the blooming simulation process under laboratoryexperiments, and that the number of precipitated grains reduced with thegrain growing of the Ti carbonitride during heating and hot working inthe blooming simulation process. The reduce of the number ofprecipitated grains deteriorated the capability of pinning the graingrowth in the parent phase, which resulted in not suppressing thecoarse-graining during the subsequent heating of the block for piercingsimulation.

To the contrary, they found that, in the simulation test without theblooming process under laboratory experiments, the Ti carbonitridefinely precipitated during the heating in the piercing process becauseno carbonitride precipitation generated within the ingot, and the Ticarbonitride pinned the grain growing in the parent phase, whereinremarkably fine grains were made.

The reason why the grains not subjected to blooming under the simulationtests under actual equipment were not finer than expected was found thatthe Ti carbonitride was already precipitated during the casting becausethe cooling rate during casting was not high enough to dissolve Ti inthe solid state.

The Ti carbonitride that precipitated during casting is apt to coarsenwith a reduced number of precipitated grains, since the precipitation iscaused at a high temperature. Therefore, the capability of pinning thegrains of the parent phase is reduced. On the other hand, if asufficient quantity of dissolved Ti is ensured during the casting withminimized precipitation of Ti carbonitride, the Ti carbonitride isfinely precipitated with an increased number of precipitated grainsduring the heating of the billet in the subsequent pipe making processbecause the precipitation is occurred at a low temperature. If thenumber of precipitated grains is large, the effect of pinning thecrystal grains of the parent phase is increased to suppress thecoarse-graining of the parent phase. Accordingly, it is extremelyimportant to properly control the cooling rate during casting.

If the cooling rate after solidification is low, the Ti carbonitrideprecipitates in a high temperature range during cooling. However, thisprecipitate in an austenite range with relatively low dislocation causesfew nucleation sites, which leads to a coarsely dispersed state. Oncecoarsely precipitated, the Ti carbonitride cannot be finely dispersedsince it is hardly dissolved in a solid phase.

If the cooling rate after solidification is set to a rate causing noprecipitation of the Ti carbonitride, the cast ingot has no Ticarbonitride, but Ti in a dissolved state. The Ti carbonitrideprecipitates at a relatively low temperature during the subsequentheating for hot working. Since, during heating, the Ti carbonitrideprecipitates at a low temperature in a bainite structure with highdislocation, the Ti carbonitride precipitates as being finely dispersedwith many nucleation sites. It was also found that an excessively highheating rate makes fine precipitation difficult because of theprecipitation in a high temperature range.

It is also effective for the sufficiently fine precipitation of Ticarbonitride to execute isothermal treatment in a proper temperaturerange during heating. The Ti carbonitride once finely precipitated ishardly coarsened, and even if blooming is executed, the effect ofsuppressing the coarse-graining can be exhibited. However, since aslight coarse-graining of Ti carbonitride is caused in the blooming, thedissolved Ti in solidification should preferably be present more thanthat in the case of executing without blooming.

Since precipitation strengthening by V or Nb makes easier to obtain highstrength, the precipitation strengthening has been frequently applied tosteel products, which require weldability in addition to high strength.However, it is better not to use the precipitation strengthening as muchas possible, since it causes serious deterioration of toughness in aheavy wall in-line heat treatment material. Particularly, Nb seriouslydeteriorates the toughness of the in-line heat treatment material.Therefore, if Nb is included, it is necessary to strictly set an upperlimit. With respect to V, it is also necessary to perform an alloydesigning in order to ensure the strength based on the transformationstrengthening and solid-solution strengthening by restricting the upperlimit of the V content, although it is not as strict as in Nb.

Further, in the case of the heavy wall material, it is difficult toobtain a homogenous metal structure in quenching treatment during thefirst stage of the heat treatment, and the toughness tends todeteriorate. Since the cooling rate is reduced in the heavy wallmaterial, it is difficult to obtain a homogenously transformedstructure. Namely, although it is successively transformed to martensiteor bainite during cooling, C is condensed to non-transformed austeniteif the diffusion of C is possible to some degree with a low coolingrate, and this part is changed to martensite or bainite with a high Ccontent or to retained austenite with a high C content after the finaltransformation. Accordingly, it is desirable to perform forced coolingto a temperature as low as possible, at a cooling rate as large aspossible.

However, there are limits to increase the cooling rate in the case ofheavy wall steel pipes. Therefore, examinations were made to develop atechnique capable of forming a homogenous structure at a cooling ratethat is attainable even in the heavy wall materials. Consequently, theinventors found that minimizing the content of the Carbon element to becondensed and also suppressing the content of Si can lead to reducingthe condensation of C in the second phase.

Based on the above findings, basic ideas of the alloy design and themanufacturing process were clarified as follows to complete the presentinvention. In the following description, “%” represents “% by mass”,unless otherwise specified.

The C content is limited to not more than 0.08%. The upper limit of theSi content is set to not more than 0.25%, preferably to not more than0.15%, and more preferably to not more than 0.10%. Ti content needs tobe controlled in a narrow range of 0.004 to 0.010% suitable toprecipitate as fine Ti carbonitride, without precipitation in thesolidification, during the subsequent billet heating. Further, anaddition of Nb is not performed in the case of an in-line heat treatmentsince it causes strength dispersion in addition to deterioration of thetoughness, and the upper limit as an impurity is preferably set to notmore than 0.005%. Since V also deteriorates the toughness, it is notadded, or should be controlled to not more than 0.08% if included.

Other elements are adjusted from the viewpoint of the balance betweenhigh strength and satisfactory toughness. For P and S that adverselyaffect the toughness, the allowable upper limit values are set,respectively. Mn, Cr. Ni, Mo and Cu should be selectively adjustedaccording to the intended strength, considering the toughness andweldability. Al is added for deoxidation. It is also effective toselectively add at least one of Ca, Mg and REM to ensure the castingcharacteristic or improve the toughness. Further, the content of N needsto be controlled in a narrow range in order to precipitate stable Ticarbonitride.

For the manufacturing process, it is important to obtain a solidifiedingot in which dissolved Ti is ensured while suppressing theprecipitation of the Ti carbonitride. The present inventors found thatthe Ti carbonitride is not precipitated immediately after solidificationif the contents of C, Ti, and N are set to the above ranges. However,since a coarse-grained Ti carbonitride is precipitated if the subsequentcooling rate is low, the cooling after solidification needs to beperformed at a specified rate or more.

Regarding the casting method, a continuous casting to a billet with acircular cross section (hereinafter, refers to “a round billet”) isideal, but a process of continuously casting to a square mold or castingthereto as an ingot, and then blooming to the round billet can beadapted. In this case, it is important to further strictly control thecooling rate after casting to ensure a sufficient quantity of dissolvedTi while suppressing precipitation of a coarse-grained TiN.

The round billet is reheated to a hot workable temperature and piercing,drawing and shaping rolling are performed thereto. If the dissolved Tiis sufficiently present, the Ti carbonitride is precipitated duringreheating. Since the precipitation temperature is relatively low,remarkably fine Ti carbonitride is precipitated, compared with theprecipitation during cooling after solidification. Since the number ofgrains of the finely precipitated Ti carbonitride is large, grainmigration during heating or holding the billet can be suppressed toprevent the coarse-graining. A quick heating causes no fineprecipitation at low temperature so that the effect of preventing thecoarse-graining cannot be obtained. Therefore, a gentle heating or aholding in a middle stage is required to promote a precipitation offine-grained Ti carbonitride.

To obtain a homogenous structure is necessary for ensuring the toughnessin the heat treatment after pipe making. It is important therefore touse steel with an adjusted chemical composition and to sufficiently coolit at a forced cooling end temperature set as low as possible. Theseideas result in improving the toughness because of preventing it fromgenerating a transformation strengthened structure with partiallyconcentrated C or retained austenite.

The present invention according to the above-mentioned basic ideasinvolves the following seamless steel pipes for line pipe (1) and (2)and the following methods of manufacturing a seamless steel pipe forline pipe (3) to (6).

(1) A heavy wall seamless steel pipe for line pipe with high strengthand increased toughness, which has a chemical composition, by mass %,that consists of C: 0.03 to 0.08%, Si: not more than 0.25%, Mn: 0.3 to2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02to 1.2%, Ti: 0.004 to 0.010%, N: 0.002 to 0.008%, and 0.0002 to 0.005%,in total, of at least one selected from Ca, Mg and REM, and the balanceFe and impurities, optionally including V: 0 to 0.08%, Nb: 0 to 0.05% orCu: 0 to 1.0%, and that P and S among impurities are not more than 0.05%and not more than 0.005% respectively.

(2) A heavy wall seamless steel pipe for line pipe with high strengthand increased toughness, which has 0.0003 to 0.01% of boron in additionto the chemical composition above.

(3) A method of manufacturing a heavy wall seamless steel pipe for linepipe with high strength and increased toughness characterized bycomprising the following steps (a) to (e):

(a) Forming a billet with a round cross section by continuous casting ofmolten steel that has the chemical composition according to (1) or (2)above.

(b) Cooling the billet to the room temperature at not less than 6°C./min of an average cooling rate between 1400 and 1000° C.

(c) Heating the billet to the temperature between 1150 and 1280° C. atnot more than 15° C./min of an average heating rate between 550 and 900°C., then piercing and rolling, to make a seamless pipe.

(d) Cooling forcedly the seamless pipe to a temperature of not higherthan 100° C. at not less than 8° C./min of an average cooling ratebetween 800 and 500° C., immediately after pipe making, or afterisothermal treating at the temperature between 850 and 1000° C.immediately in succession with pipe making, or after heating to thetemperature between 850 and 1000° C. after once cooling in successionwith pipe making.

(e) Tempering the seamless pipe at the temperature between 500 and 690°C.

(4) A method of manufacturing a heavy wall seamless steel pipe for linepipe with a high strength and increased toughness characterized bycomprising the following steps (a) to (f):

(a) Forming a bloom or slab with a square cross section by continuouscasting of molten steel that has the chemical composition according to(1) or (2) above.

(b) Cooling the bloom or slab to the room temperature at not less than8° C./min of an average cooling rate between 1400 and 1000° C.

(c) Heating the bloom or slab to a temperature between 1150 and 1280° C.at not more than 15° C./min of an average heating rate between 550 and900° C., and then cooling to the room temperature, to form a billet witha round cross section by forging and/or rolling.

(d) Heating the billet to the temperature between 1150 and 1280° C.,then piercing and rolling, to make a seamless pipe.

(e) Cooling forcedly the seamless pipe to a temperature of not higherthan 100° C. at not less than 8° C./min of an average cooling ratebetween 800 and 500° C., immediately after pipe making, or afterisothermal treating at the temperature between 850 and 1000° C.immediately in succession with pipe making, or after heating to thetemperature between 850 and 1000° C. after once cooling in successionwith pipe making.

(f) Tempering the seamless pipe at the temperature between 500 and 690°C.

(5) A method of manufacturing a heavy wall seamless steel pipe for linepipe with a high strength and increased toughness characterized bycomprising the following steps (a) to (e):

(a) Forming a billet with a round cross section by continuous casting ofa molten steel that has the chemical composition according to (1) or (2)above.

(b) Cooling the billet to the room temperature at not less than 6°C./min of an average cooling rate between 1400 and 1000° C.

(c) Isothermal treating the billet during not less than 15 minutes atthe temperature between 550 and 1000° C., and heating to the temperaturebetween 1150 and 1280° C., then piercing and rolling, to make a seamlesspipe.

(d) Cooling forcedly the seamless pipe to a temperature of not higherthan 100° C. at not less than 8° C./min of an average heating ratebetween 800 and 500° C., immediately after pipe making, or afterisothermal treating at the temperature between 850 and 1000° C.immediately in succession with pipe making, or after heating to thetemperature between 850 and 1000° C. after once cooling in successionwith pipe making.

(e) Tempering the seamless pipe at the temperature between 500 and 690°C.

(6) A method of manufacturing a heavy wall seamless steel pipe for linepipe with a high strength and increased toughness characterized bycomprising the following steps (a) to (f):

(a) Forming a bloom or slab with a square cross section by continuouscasting of a molten steel that has the chemical composition according to(1) or (2) above.

(b) Cooling the bloom or slab to the room temperature at not less than8° C./min of an average cooling rate between 1400 and 1000° C.

(c) Isothermal treating the bloom or slab during not less than 15minutes at the temperature between 550 and 1000° C., and heating to thetemperature between 1150 and 1280° C., then forging and/or rolling, toform a billet with a round cross section, and then cooling to the roomtemperature.

(d) Heating the billet to the temperature of 1150 to 1280° C., thenpiercing and rolling, to make a seamless pipe.

(e) Cooling forcedly the seamless pipe to a temperature of not higherthan 100° C. at not less than 8° C./min of an average cooling ratebetween 800 and 500° C., immediately after pipe making, or afterisothermal treating at the temperature between 850 and 1000° C.immediately in succession with pipe making, or after heating to thetemperature between 850 and 1000° C. after once cooling in successionwith pipe making.

(f) Tempering the seamless pipe at the temperature between 500 and 690°C.

BEST MODE FOR CARRYING OUT THE INVENTION

1. Chemical Composition of Steel Pipe of the Present Invention

The reason for limiting the chemical compositions of the steel pipe ofthe present invention will be described as follows. As described above,“%” showing the content (or concentration) of a chemical compositionmeans “% by mass”.

C: 0.03 to 0.08%

C is an important element for ensuring the strength of steel. A contentof not less than 0.03% is needed in order to improve the hardenabilityand strength in a heavy wall material. Since a content exceeding 0.08%causes deterioration of toughness, the C content is set to 0.03 to0.08%.

Si: 0.25% or less

Si has an effect as a deoxidizer in steel production, but it is betterto add as little as possible. Because it seriously deteriorates thetoughness, particularly, of a heavy wall material. If the content of Siexceeds 0.25%, the toughness of the heavy wall material is remarkablydeteriorated. Therefore, the content is set to 0.25% or less if it isadded as the deoxidizer. A content of 0.15% or less enables furtherimprovement in toughness. The content is most desirably controlled toless than 0.10%. Although it is difficult to extremely reduce Si asimpurity from the point of the steel making process, extremelysatisfactory toughness can be obtained if the content is limited to lessthan 0.05%.

Mn: 0.3 to 2.5%

Mn needs to be included in a relatively large quantity since it enhancesthe hardenability and therefore strengthens the center even in a heavywall material and also enhances the toughness. A content of less than0.3% cannot provide these effects, and a content exceeding 2.5% causesdeterioration of the HIC resisting characteristic, therefore, the Mncontent is set to 0.3 to 2.5%.

Al: 0.001 to 0.10%

Al is added as a deoxidizer in steel making. In order to obtain thiseffect, it needs to be added so as to have a content of not less than0.001%. On the other hand, if the content of Al exceeds 0.10%, theinclusions are clustered, thereby deteriorating the toughness, andsurface defects are frequently generated during the bevel face workingof pipe ends. Therefore, the content of Al is set to 0.001 to 0.10%.From the point of preventing the surface defects, it is desirable toprovide an upper limit, and the upper limit is preferably set to 0.03%and more preferably to 0.02%. Since a high deoxidation effect by theaddition of Si cannot be expected in the steel pipe of the presentinvention, the lower limit of Al content is preferably set to 0.010% forsufficient deoxidation.

Cr: 0.02 to 1.0%

Cr is an element that improves the hardenability and strength of steelin a heavy wall material. The effect becomes remarkable when 0.02% ormore of Cr is included. However, since an excessive content thereofcauses some deterioration of toughness, the content is limited to 1.0%or less.

Ni: 0.02 to 1.0%

Ni is an element that improves the hardenability and strength of steelin a heavy wall material. The effect becomes remarkable when 0.02% ormore of Ni is included. However, since Ni is an expensive element, andthe effect is saturated if it is excessively included, the upper limitthereof is set to 1.0%.

Mo: 0.02 to 1.2%

Mo is an element that improves the strength of steel by transformationstrengthening and solid-solution strengthening. The effect becomesremarkable when 0.02% or more of Mo is included. However, since anexcessive addition thereof causes deterioration of the toughness, theupper limit is set to 1.2%.

Ti: 0.004 to 0.010%

The content of Ti needs to be controlled in a narrow range of 0.004% to0.010% suitable to precipitate as fine Ti carbonitride, withoutprecipitation in the solidification, during the subsequent billetheating. If the content is less than 0.004%, a sufficient number ofprecipitated grains of the Ti carbonitride cannot be ensured, and if itexceeds 0.010%, the Ti carbonitride is coarsely precipitated in thecooling after solidification. Therefore, a proper content of Ti is 0.004to 0.010%.

N: 0.002 to 0.008%

N needs to be included in a content of 0.002% or more to ensure finelydispersed Ti carbonitride. Since a content exceeding 0.008% results inprecipitation of coarse-grained Ti carbonitride in the solidification,the content needs to be controlled in a narrow range of 0.002 to 0.008%.

V: 0 to 0.08%

V is an element whose content is to be determined depending on thebalance between strength and toughness. If sufficient strength can beensured by other alloy elements, increased toughness can be obtainedwithout the addition thereof. When it is added as a strength improvingelement, the content is preferably set to 0.02% or more. Since thetoughness is seriously deteriorated if the content exceeds 0.08%, theupper limit of the content is set to 0.08% if added.

Nb: 0 to 0.05%

Nb is remarkably effective for suppressing the coarse-graining duringheating for quenching in the case of an off-line heat treatment. Inorder to obtain this effect, Nb is desirably included in a content of0.005% or more. However, if the content of Nb exceeds 0.05%, acoarse-grained carbonitride is precipitated to deteriorate thetoughness. Therefore, the upper limit is set to 0.05%.

In the case of an in-line heat treatment, basically, it is better not toadd Nb since the Nb carbonitride is inhomogeneously precipitated, whichincreases the strength dispersion as well as deterioration of thetoughness. When the content exceeds 0.005%, the strength dispersion isremarkable and problematic in manufacturing. Therefore, when applyingthe in-line heat treatment, the allowable upper limit should be set to0.005%.

Cu: 0 to 1.0%

Cu does not need to be added. However, it can be added, if improvementin the HIC resisting characteristic (hydrogen-induced cracking resistingcharacteristic) is intended, since it has an effect of improving the HICresisting characteristic. The minimum content that improves the HICcharacteristic is 0.02%. Since the effect is saturated even with acontent that exceeds 1.0%, the content may be set to 0.02 to 1.0% ifadded.

Ca, Mg, and REM: 00002 to 0.005%, in total of at least one selectedtherefrom.

These elements are added for the purpose of improving the toughness andthe corrosion resistance by shape controlling of inclusions and for thepurpose of suppressing nozzle clogging in casting to improve the castingcharacteristic. In order to obtain such effects, a content of 0.0002% ormore, in total of at least one selected therefrom is needed. If thecontent exceeds 0.005%, in total of at least one selected therefrom, notonly the effects are saturated, but also the inclusions are easilyclustered, thereby somewhat deteriorating the toughness and the HICresisting characteristic. Therefore, when one of these elements areadded, each content is set to 0.0002 to 0.005%, and when two or moreselected therefrom are added, the total content is set to 0.0002 to0.005%. REM means 17 elements that include lanthanoide elements, Y andSc.

B: 0.0003 to 0.01%

B does not need to be added. However, since addition thereof leads toimprovement in hardenability even if it is a trace, the addition iseffective when further high strength is needed. In order to obtain thiseffect, a content of 0.0003% or more is desirable. However, since anexcessive addition thereof causes deterioration of the toughness, thecontent of B is set to 0.01% or less if added.

The steel pipe for line pipe of the present invention contains the abovechemical composition and the balance Fe and impurities. The upper limitsof content of P and C among the impurities should be controlled asfollows.

P: Not more than 0.05%

P is an impurity element that deteriorates the toughness, and thecontent is preferably as little as possible. Since a content exceeding0.05% causes remarkable deterioration of the toughness, the allowableupper limit is set to 0.05%. The content of P is preferably 0.02% orless and, further preferably, 0.01% or less.

S: Not more than 0.005%

S is also an impurity element that deteriorates the toughness, and thecontent is preferably as little as possible. Since a content exceeding0.005% causes remarkable deterioration of the toughness, the allowableupper limit is set to 0.005%. The content of S is preferably 0.003% orless and, further preferably, 0.001% or less.

2. Manufacturing Method

Suitable manufacturing conditions of the manufacturing method of thepresent invention will be described as follows.

(1) Casting and Cooling after Solidification

Steel is refined in a basic oxygen furnace or the like so as to have theabove composition followed by casting and solidification to obtain abloom. At this time, it is important to obtain a solidified ingot inwhich precipitation of the Ti carbonitride is suppressed. If thecontents of C, Ti and N are restricted as described above, basically,the Ti carbonitride is not precipitated during solidification. However,if the subsequent cooling rate is low, a coarse-grained Ti carbonitrideprecipitates, therefore, the cooling must be performed at not less thana specified cooling rate.

A continuous casting to a round billet shape is ideal for themanufacturing process, however, a process for continuously casting to asquare mold or casting thereto as an ingot and then blooming to theround billet can be performed. In this case, it is important to furtherstrictly control the cooling rate after solidification to suppress theprecipitation of a coarse-grained TiN.

An average cooling rate in a temperature range of 1400 to 1000° C.,where the Ti carbonitride is apt to be generated after solidification,is required to be not less than 6° C./min in the case of casting to theround billet and to be not less than 8° C./min in the case of executingthe blooming. The average cooling rate is more preferably set to be notless than 8° C./min in the case of casting to the round billet and to benot less than 10° C./min in the case of executing the blooming. In eachcase, no upper limit is provided since the larger average cooling rateis more desirable.

The cooling rate of the bloom varies depending on portions of the bloom.In the case of continuous casting to the circular mold, the cooling rateis controlled at a place distant from a center by the distance of ½ ofthe radius. In the case of continuously casting to a square mold, thecooling rate is controlled at a middle position between the center ofgravity and the surface on a line passing the center of gravity of thesquare in parallel to the long sides thereof. The temperature can bemeasured by attaching a thermocouple, or instead, by numericalsimulation corrected with the temperature history of the surface.

(2) Working of Billet or Ingot

The round billet is reheated to a hot workable temperature, andpiercing, drawing and shaping rolling are performed thereto. The bloomor slab cast into a square cross section is reheated and then made intoa round billet by forging or/and rolling, and piercing, drawing andshaping rolling are then performed thereto.

A reheating temperature is required to be 1150° C. or higher since hotdeformation resistance is increased at a lower temperature than 1150°C., which increases flaws. The upper limit thereof is set to 1280° C.,since a temperature exceeding 1280° C. leads to an excessive increase inthe heating fuel raw unit, and a reduction in yield by increased scaleloss, which uneconomically shortened the life of the heating furnace,and the like. Since a lower heating temperature makes finer grains andincreases the toughness, a preferable heating temperature is 1200° C. orlower.

When the dissolved Ti is sufficiently present, the Ti carbonitride isprecipitated during reheating, however, the precipitation occurs at arelatively low temperature, unlike the precipitation during the coolingafter solidification. Therefore, the precipitated Ti carbonitride ismuch more fine-grained than the one during the cooling aftersolidification. An increased number of fine-grained Ti carbonitride areformed, and this suppresses the grain migration during heating thebillet to prevent the coarse-graining. However, since rapid heatingdisables minute precipitation at a low temperature, the effect ofpreventing coarse-graining cannot be obtained. It is effective for thepromotion of the minute precipitation at a low temperature during thereheating that the average heating rate should be 15° C./min or less ata temperature between 550 and 900° C. or that the isothermal treatmentshould be executed for 15 minutes or more at a temperature between 550and 1000° C.

The piercing, drawing and shaping rolling can be executed in themanufacturing conditions for a general seamless steel pipe.

3. Heat Treatment after Pipe Making

In the heat treatment after pipe making, to obtain a homogeneousstructure is needed in order to ensure toughness. The quenchingtreatment is based on the in-line heat treatment of executing quenching,without once cooling to room temperature, in succession with hotrolling. However, if the reheating and the quenching are performed aftercooling, finer grains are made, improving the toughness. When thequenching is executed in succession with isothermal treatment in anisothermal furnace after the end of hot work, a steel pipe withminimized strength dispersion can be obtained.

High strength and high toughness can be obtained more easily in a heavywall material if the cooling rate in the quenching is set higher. As thecooling rate gets closer to a theoretically limited cooling rate, higherstrength and higher toughness can be obtained. The necessary averagecooling rate is 8° C./sec or more at a temperature between 800 and 500°C., more preferably, 10° C./sec or more, and most preferably not lessthan 15° C./sec.

The cooling end temperature is also important for ensuring excellenttoughness, in addition to the cooling rate. It is important to use asteel with an adjusted chemical composition and forcedly cool it to anend temperature of not higher than 100° C. The forced cooling iscontinuously performed, preferably to 80° C. or lower, more preferablyto 50° C. or lower, and most preferably to 30° C. or lower. According tothis, the generation of transformation strengthened structure in which Cis partially concentrated or retained austenite can be prevented,therefore, the toughness is significantly improved.

After the quenching, tempering is performed at a temperature between 500and 700° C. The tempering is performed in order to adjust the strengthand improving the toughness. The holding time at the temperingtemperature may be properly determined according the wall thickness orthe like of the steel pipe, and is generally set to about 10 to 120minutes.

EXAMPLE

Steels that have the chemical compositions shown in Table 1 were moltenin a converter. Two methods for manufacturing a round billet wereadopted: one is a method for casting to a continuous mold with a roundcross section, and the other is a manufacturing method for casting to asquare mold and then blooming. The manufacturing conditions in thecasting to the round continuous casting mold are shown in Tables 2 and3. The solidification process is represented as “RCC”. The process forcasting to the square mold is represented as “BLCC”, and themanufacturing conditions thereof are shown in Tables 4 and 5.

Round billets were heated in the pipe making heating conditions shown inTables 2 to 5, and hollow pipes were produced by use of a feed rollpiercing machine. The hollow pipes were finish-rolled by use of amandrel mill and a sizer, whereby steel pipes having wall thickness of30 mm to 50 mm were obtained. Thereafter, these pipes were cooled in thequenching conditions described in Tables 2 to 5. Namely, after pipemaking, any one of the following three processes were adopted: the firstone is cooling immediately; the second one is charging immediately to areheating furnace for isothermal treatment and then quenching; and thelast one is cooling once to room temperature and reheating and thencooling again. Thereafter, tempering was executed in the conditionsdescribed in Tables 2 to 5 to obtain the finished products.

A JIS (Japan Industrial Standard) No. 12 tensile test piece for tensiletest was prepared from each of the resulting steel pipes to measuretensile strength (TS) and yield strength (YS). The tensile test wascarried out according to JIS Z 2241. As an impact test piece, a V notchtest piece of 10 mm×10 mm, 2 mm was prepared from the longitudinaldirection of the heavy wall center according to No. 4 test piece of JISZ 2202, and subjected to the test.

In Test No. 1 of Table 2, two examples of branch numbers of 1 and 2 aredescribed. Steel A, the invention, is used in 1-1 and 1-2, and themanufacturing condition of 1-1 is within the range restricted by thepresent invention, where increased toughness is obtained. On the otherhand, the manufacturing condition of 1-2 is deviated from themanufacturing process defined by the present invention with anexcessively high heating rate for pipe making, where increased toughnesscannot be obtained. Each of Test Nos. 2 to 24 also has branch numbers 1and 2, and the same steel grade is used in the same test number. Themanufacturing condition of each branch number 1 is within the rangerestricted by the present invention, where increased toughness can beobtained. On the other hand, the manufacturing condition of each branchnumber 2 is deviated from the manufacturing process defined by thepresent invention, where increased toughness cannot be obtained.

In Tables 4 and 5, the same steel grade is also used in one test number,and each branch number 1 corresponds to the manufacturing process withinthe range restricted by the present invention, where increased toughnessis obtained. On the other hand, since each branch number 2 is deviatedfrom the manufacturing process defined by the present invention, whereincreased toughness is not obtained.

Test Nos. 25 to 30 are examples of comparative steels that are deviatedfrom the alloy composition range restricted by the present invention.Each of the steels is insufficient in toughness, and has insufficientperformances as a line pipe requiring increased thickness and hightoughness. TABLE 1 Chemical Composition [mass % Fe: balance] Steel C SiMn P S Cr Ni Mo Ti sol.Al N A 0.05 0.07 1.77 0.008 0.0012 0.50 0.12 0.160.006 0.025 0.0036 B 0.05 0.08 1.46 0.006 0.0013 0.41 0.06 0.25 0.0080.015 0.0053 C 0.03 0.07 1.77 0.012 0.0009 0.25 0.12 0.13 0.010 0.0270.0035 D 0.06 0.05 1.19 0.011 0.0009 0.47 0.16 0.24 0.007 0.027 0.0048 E0.03 0.09 1.54 0.010 0.0013 0.48 0.14 0.10 0.006 0.019 0.0065 F 0.060.08 1.69 0.007 0.0011 0.27 0.17 0.14 0.008 0.029 0.0039 G 0.07 0.091.53 0.006 0.0010 0.36 0.26 0.16 0.007 0.023 0.0057 H 0.07 0.08 1.600.006 0.0005 0.37 0.12 0.08 0.010 0.023 0.0026 I 0.07 0.05 1.53 0.0080.0014 0.07 0.16 0.22 0.006 0.023 0.0040 J 0.03 0.07 1.60 0.009 0.00080.33 0.27 0.23 0.007 0.024 0.0026 K 0.07 0.06 1.49 0.009 0.0009 0.390.10 0.07 0.009 0.020 0.0067 L 0.03 0.10 1.46 0.008 0.0011 0.44 0.140.16 0.007 0.027 0.0040 M 0.05 0.09 1.75 0.007 0.0006 0.35 0.19 0.170.010 0.017 0.0048 N 0.04 0.06 1.43 0.005 0.0010 0.35 0.15 0.29 0.0090.016 0.0050 O 0.04 0.07 1.31 0.011 0.0009 0.44 0.24 0.31 0.010 0.0150.0025 P 0.06 0.06 1.47 0.009 0.0005 0.31 0.05 0.18 0.008 0.024 0.0068 Q0.03 0.06 1.35 0.006 0.0005 0.42 0.12 0.21 0.006 0.020 0.0055 R 0.030.05 1.55 0.011 0.0007 0.23 0.18 0.19 0.009 0.029 0.0063 S 0.06 0.071.44 0.011 0.0014 0.33 0.16 0.19 0.009 0.017 0.0027 T 0.05 0.08 1.690.011 0.0005 0.38 0.11 0.06 0.007 0.026 0.0045 U 0.04 0.08 1.42 0.0090.0009 0.32 0.20 0.30 0.007 0.020 0.0020 V 0.07 0.09 1.30 0.006 0.00090.59 0.14 0.26 0.006 0.028 0.0041 W 0.04 0.07 1.72 0.012 0.0010 0.500.20 0.22 0.008 0.018 0.0039 X 0.03 0.08 1.51 0.006 0.0009 0.47 0.350.33 0.009 0.025 0.0038 AA 0.11 0.09 1.36 0.008 0.0012 0.49 0.11 0.170.008 0.029 0.0036 BB 0.06 0.34 1.47 0.009 0.0011 0.49 0.10 0.28 0.0070.023 0.0033 CC 0.04 0.06 1.59 0.008 0.0070 0.52 0.15 0.25 0.006 0.0270.0035 DD 0.05 0.08 1.65 0.009 0.0009 0.57 0.14 0.23 0.019 0.028 0.0032EE 0.04 0.08 1.80 0.008 0.0010 0.46 0.10 0.21 0.007 0.023 0.0036 FF 0.040.08 1.75 0.009 0.0008 0.45 0.10 0.19 0.009 0.020 0.0042 ChemicalComposition [mass % Fe: balance] Steel V Cu Nb B Ca Mg REM A 0.04 0.00 —— 0.0007 — — The invention B 0.04 0.00 — — 0.0009 — — C 0.01 0.00 — —0.0025 — — D 0.01 0.00 — — 0.0022 — — E 0.00 0.00 — — 0.0011 — — F 0.030.00 — — 0.0010 — — G 0.00 0.25 — — 0.0024 — — H 0.00 0.00 — 0.00210.0015 — — I 0.03 0.33 — — 0.0020 — — J 0.03 0.00 — — 0.0008 — — K 0.000.31 — — 0.0014 — — L 0.03 0.32 — — 0.0012 — — M 0.00 0.00 — — 0.0016 —— N 0.04 0.00 — — 0.0027 0.0008 — O 0.04 0.00 — 0.0011 0.0025 — 0.0015 P0.03 0.00 — — 0.0019 0.0012 — Q 0.01 0.25 — — 0.0013 0.0011 — R 0.000.00 — — 0.0017 — — S 0.00 0.00 — 0.0022 0.0007 — — T 0.04 0.23 — —0.0020 — — U 0.04 0.00 — 0.0011 0.0017 — — V 0.02 0.30 0.022 — 0.0029 —— W 0.00 0.18 0.008 0.0015 0.0020 0.0008 0.0007 X 0.03 0.31 0.019 —0.0027 — — AA 0.04 0.00 — 0.0014 0.0008 — — The comparative BB 0.03 0.00— 0.0013 0.0011 — — CC 0.04 0.00 — 0.0013 0.0008 — — DD 0.02 0.00 —0.0016 0.0008 — — EE 0.11 0.00 — 0.0015 0.0012 — — FF 0.04 0.00 — 0.0015— — —

TABLE 2 Quenching after pipe making Charging Cooling to room Heatingduring pipe making to a reheating temperature and chargingSolidification Holding in a furnace before to a reheating furnaceCooling Heating Heating middle stage quenching before quenching RateTemp. Rate Temp. Time Temp. Time Temp. Time Test No. Steel Process (°C./min) (° C.) (° C.) (° C.) (min) (° C.) (min) (° C.) (min)  1-1 A RCC10.6 1250 — 600 90 950 10 — —  1-2 A RCC 15.1 1200 16.5 — — 950 10 — — 2-1 B RCC 11.7 1250 9.4 — — 900 20 — —  2-2 B RCC 4.5 1150 7.9 — — 90020 — —  3-1 C RCC 10.2 1150 — 800 90 950 10 — —  3-2 C RCC 12.1 1300 6.5— — 950 10 — —  4-1 D RCC 9.7 1150 5.6 — — 950 10 — —  4-2 D RCC 12.51100 16.4 400 90 950 10 — —  5-1 E RCC 14.1 1200 9.9 — — 950 10 — —  5-2E RCC 10.7 1250 7.7 — — 950 10 — —  6-1 F RCC 8.2 1200 7.1 — — 980  5 ——  6-2 F RCC 13 1150 — 750 120  980  5 — —  7-1 G RCC 11.8 1200 9.0 — —950 10 — —  7-2 G RCC 13.6 1310 6.3 — — 950 10 — —  8-1 H RCC 8.4 12506.2 — — 940 10 — —  8-2 H RCC 15 1200 7.4 — — 940 10 — —  9-1 I RCC 10.91250 — 650 30 950 10 — —  9-2 I RCC 4.5 1100 — 700 120  950 10 — — 10-1J RCC 14.4 1100 9.9 — — 950 10 — — 10-2 J RCC 10.9 1100 6.7 — — 950 10 —— 11-1 K RCC 15.7 1250 8.1 — — 950 10 — — 11-2 K RCC 12.7 1200 15.7 — —950 10 — — 12-1 L RCC 11.9 1150 7.7 — — 950 10 — — 12-2 L RCC 12.4 12005.8 — — 1080  10 — — 13-1 M RCC 8.7 1250 6.9 — — 950 30 — — 13-2 M RCC12.6 1100 — 600 60 780 30 — — 14-1 N RCC 10.1 1250 8.7 — — — — — — 14-2N RCC 13.6 1200 — 850 60 — — — — Quenching after Toughness pipe makingFracture Temp. Strength Appearance Thickness Cooling after TemperingYield Transition of Rate cooling Temp. Time Strength Temp. (FATT) pipewall Test No. (° C./sec) (° C.) (° C.) (° C.) (MPa) (° C.) (mm) Note (*) 1-1 8.4 37 590 20 772 −38 42 I  1-2 11.6 55 630 30 752 −7 42 C  2-111.7 56 600 30 663 −58 40 I  2-2 9.7 59 600 30 620 −19 40 C  3-1 15.6 26590 20 586 −45 35 I  3-2 10.1 60 610 20 568 0 35 C  4-1 14.6 50 640 30605 −51 35 I  4-2 10.9 57 560 20 582 −4 35 C  5-1 11.5 62 590 20 611 −3345 I  5-2 5.5 47 590 30 567 −16 45 C  6-1 14.8 53 550 20 630 −60 35 I 6-2 8.3 200 600 10 655 −18 35 C  7-1 9.2 31 610 20 682 −58 30 I  7-219.4 51 640 10 690 −5 30 C  8-1 17.8 46 600 10 755 −39 32 I  8-2 4.1 48620 30 709 2 32 C  9-1 18 30 570 10 617 −51 32 I  9-2 8.9 40 620 20 612−15 32 C 10-1 19.1 53 640 20 647 −31 30 I 10-2 14.3 179 580 10 641 −2230 C 11-1 9.2 57 640 10 654 −42 50 I 11-2 8.7 50 550 10 643 2 50 C 12-110.6 51 580 20 605 −35 45 I 12-2 9.5 55 610 10 624 −12 45 C 13-1 11.8 31560 20 706 −42 40 I 13-2 13.2 41 610 20 703 −9 40 C 14-1 16 40 640 30640 −54 35 I 14-2 4.3 26 560 10 591 −2 35 C(*) Note: The symbol I shows the invention and the symbol C shows thecomparative.

TABLE 3 Quenching after pipe making Charging Cooling to room Heatingduring pipe making to a reheating temperature and chargingSolidification Holding in a furnace before to a reheating furnaceCooling Heating Heating middle stage quenching before quenching RateTemp. Rate Temp. Time Temp. Time Temp. Time Test No. Steel Process (°C./min) (° C.) (° C.) (° C.) (min) (° C.) (min) (° C.) (min) 15-1 O RCC15.1 1100 6.8 — — — — — — 15-2 O RCC 15.8 1150 5.3 — — — — — — 16-1 PRCC 13.2 1150 — 800 90 — — — — 16-2 P RCC 14.6 1150 — 700 60 — — — —17-1 Q RCC 8.3 1150 8.7 — — — — — — 17-2 Q RCC 15 1150 — 800 60 — — — —18-1 R RCC 13.4 1100 — 700 90 — — — — 18-2 R RCC 5.2 1200 9.3 — — — — —— 19-1 S RCC 13.9 1200 9.3 — — — — — — 19-2 S RCC 15.3 1310 5.1 — — — —— — 20-1 T RCC 10.9 1100 — 850 90 — — — — 20-2 T RCC 13.4 1250 19.7 — —— — — — 21-1 U RCC 11.8 1200 7.9 — — — — — — 21-2 U RCC 14 1200 18.2 30030 — — — — 22-1 V RCC 10.3 1250 — 750 30 — — 920 15 22-2 V RCC 14.6 12509.1 — — — — 920 15 23-1 W RCC 11.4 1150 — 600 120  — — 920 30 23-2 W RCC11.9 1250 — 800 90 — — 920 30 24-1 X RCC 13.6 1250 6.8 — — — — 920 2024-2 X RCC 9.5 1250 5.6 — — — — 1100  20 25 AA RCC 11.8 1250 8.0 — — — —920 15 26 BB RCC 11.7 1250 8.1 — — — — 920 15 27 CC RCC 10.6 1250 7.6 —— — — 920 15 28 DD RCC 12.3 1250 7.2 — — — — 920 15 29 EE RCC 11.2 12508.6 — — — — 920 15 30 FF RCC 11.6 1250 7.5 — — — — 920 15 Quenchingafter Toughness pipe making Fracture Temp. Strength Appearance ThicknessCooling after Tempering Yield Transition of Rate cooling Temp. TimeStrength Temp. (FATT) pipe wall Test No. (° C./sec) (° C.) (° C.) (° C.)(MPa) (° C.) (mm) Note (*) 15-1 14.5 50 580 30 745 −31 30 I 15-2 19.6163 640 20 766 −2 30 C 16-1 9.3 36 640 10 586 −43 40 I 16-2 14.3 30 40030 573 −9 40 C 17-1 13.3 60 630 20 577 −47 30 I 17-2 18.6 45 750 30 535−10 30 C 18-1 16.3 53 630 10 530 −42 35 I 18-2 15.1 52 630 30 519 −24 35C 19-1 13.4 37 570 20 720 −51 30 I 19-2 18.9 44 560 10 698 −3 30 C 20-18.4 45 640 10 679 −47 40 I 20-2 13.9 39 620 30 673 2 40 C 21-1 11.3 43590 20 751 −35 35 I 21-2 17.1 40 550 10 746 −14 35 C 22-1 9.6 34 620 30875 −36 43 I 22-2 11.7 210 580 20 872 8 43 C 23-1 12.4 45 560 20 888 −4240 I 23-2 3.7 45 640 20 837 16 40 C 24-1 12.9 48 630 30 889 −38 40 I24-2 12.1 28 610 30 900 −19 40 C 25 10.6 34 600 30 860 −12 40 C 26 10.131 600 30 840 −11 40 C 27 11.2 27 600 30 825 −12 40 C 28 9.8 28 600 30914 −6 45 C 29 10.4 30 600 30 870 −15 45 C 30 10.3 31 600 30 886 −17 45C(*) Note: The symbol I shows the invention and the symbol C shows thecomparative.

TABLE 4 Quenching after pipe making Cooling to room temperature andHeating during blooming Pipe Charging to a charging to a SolidificationHolding in a making reheating furnace reheating furnace Cooling HeatingHeating middle stage Heating before quenching before quenching RateTemp. Rate Temp. Time Temp. Temp. Time Temp. Time Test No. Steel Process(° C./min) (° C.) (° C.) (° C.) (min) (° C.) (° C.) (min) (° C.) (min) 1-1 A BLCC 15.0 1150 8.3 — — 1250 950 10 — —  1-2 A BLCC 14.3 1200 6.1— — 1310 950 10 — —  2-1 B BLCC 15.2 1250 7.8 — — 1200 950 10 — —  2-2 BBLCC 16.1 1100 7.8 — — 1250 950 10 — —  3-1 C BLCC 15.1 1150 8.0 — —1150 950 10 — —  3-2 C BLCC 16.0 1200 17.1 350  30 1150 950 10 — —  4-1D BLCC 13.8 1200 6.7 — — 1100 950 10 — —  4-2 D BLCC 11.7 1310 6.0 — —1200 950 10 — —  5-1 E BLCC 11.7 1200 4.8 — — 1250 950 10 — —  5-2 EBLCC 14.4 1150 7.9 — — 1250 950 10 — —  6-1 F BLCC 12.0 1200 5.6 — —1200 950 10 — —  6-2 F BLCC 16.3 1150 7.8 — — 1150 760 10 — —  7-1 GBLCC 13.1 1150 5.8 — — 1250 950 10 — —  7-2 G BLCC 13.6 1200 7.1 — —1100 950 10 — —  8-1 H BLCC 14.7 1200 6.4 — — 1250 950 10 — —  8-2 HBLCC 7.1 1150 5.9 — — 1150 950 10 — —  9-1 I BLCC 15.5 1250 6.7 — — 1150950 10 — —  9-2 I BLCC 11.3 1200 5.7 — — 1320 950 10 — — 10-1 J BLCC12.3 1250 6.3 — — 1250 950 10 — — 10-2 J BLCC 12.3 1200 16.8 — — 1150950 10 — — 11-1 K BLCC 15.5 1250 7.7 — — 1200 950 10 — — 11-2 K BLCC16.3 1250 — 750 120 1200 950 10 — — 12-1 L BLCC 15.8 1100 8.2 — — 1200950 10 — — 12-2 L BLCC 15.1 1250 6.8 — — 1100 1090  10 — — 13-1 M BLCC17.3 1250 8.1 — — 1150 950 10 — — 13-2 M BLCC 6.7 1200 5.2 — — 1250 95010 — — 14-1 N BLCC 14.1 1100 7.3 — — 1200 — — — — 14-2 N BLCC 10.0 11505.8 — — 1250 — — — — Toughness Quenching after Fracture pipe makingAppearance Temp. Strength Transition Thickness Cooling after TemperingYield Temp. of Rate cooling Temp. Time Strength (FATT) pipe wall TestNo. (° C./sec) (° C.) (° C.) (° C.) (MPa) (° C.) (mm) Note (*)  1-1 1744 580 20 779 −30 35 I  1-2 16.9 35 580 20 778 4 35 C  2-1 11.3 42 59010 649 −54 45 I  2-2 4.9 60 570 20 607 −13 45 C  3-1 10.1 65 640 30 597−33 30 I  3-2 19.4 38 580 10 583 −21 30 C  4-1 11 48 560 20 580 −31 35 I 4-2 17.2 38 610 10 614 −17 35 C  5-1 17.1 27 600 20 606 −40 32 I  5-24.4 26 560 10 555 −19 32 C  6-1 11.8 41 590 20 662 −27 40 I  6-2 11.4 36630 10 631 −9 40 C  7-1 15.5 42 600 30 714 −28 35 I  7-2 11.2 179 560 20712 −7 35 C  8-1 17.7 69 560 30 771 −27 33 I  8-2 13.1 45 590 30 780 −133 C  9-1 16.5 33 600 30 616 −48 35 I  9-2 12 66 560 10 632 4 35 C 10-117.4 56 620 30 651 −43 30 I 10-2 19.9 39 600 30 640 0 30 C 11-1 8.8 33580 20 633 −50 50 I 11-2 8.2 192 550 30 666 −3 50 C 12-1 15.3 26 580 20612 −46 30 I 12-2 18.5 37 640 20 616 −23 30 C 13-1 16.2 71 550 30 683−51 35 I 13-2 10.1 66 550 10 680 −3 35 C 14-1 19.3 55 630 20 607 −46 30I 14-2 19 41 380 20 625 −23 30 C(*) Note: The symbol I shows the invention and the symbol C shows thecomparative.

TABLE 5 Quenching after pipe making Cooling to room temperature andHeating during blooming Pipe Charging to a charging to a SolidificationHolding in a making reheating furnace reheating furnace Cooling HeatingHeating middle stage Heating before quenching before quenching RateTemp. Rate Temp. Time Temp. Temp. Time Temp. Time Test No. Steel Process(° C./min) (° C.) (° C.) (° C.) (min) (° C.) (° C.) (min) (° C.) (min)15-1 O BLCC 16.9 1250 7.6 — — 1150 — — — — 15-2 O BLCC 15.5 1150 19.1250 30 1150 — — — — 16-1 P BLCC 14.1 1200 6.4 — — 1100 — — — — 16-2 PBLCC 17.5 1100 7.7 — — 1200 — — — — 17-1 Q BLCC 12.0 1200 6.2 — — 1250 —— — — 17-2 Q BLCC 17.3 1100 — 800 60 1150 — — — — 18-1 R BLCC 14.8 11007.5 — — 1250 — — — — 18-2 R BLCC 6.9 1250 5.6 — — 1100 — — — — 19-1 SBLCC 11.8 1200 6.8 — — 1200 — — — — 19-2 S BLCC 17.9 1330 — 750 120 1250 — — — — 20-1 T BLCC 14.3 1250 6.4 — — 1250 — — — — 20-2 T BLCC 16.51250 8.5 — — 1200 — — — — 21-1 U BLCC 13.5 1250 7.5 — — 1250 — — — —21-2 U BLCC 10.4 1150 20.8 — — 1150 — — — — 22-1 V BLCC 17.0 1200 — 60060 1200 — — 910 30 22-2 V BLCC 15.5 1150 — 700 60 1250 — — 1090  30 23-1W BLCC 15.6 1150 8.4 — — 1200 — — 920 20 23-2 W BLCC 15.3 1250 7.6 — —1250 — — 920 20 24-1 X BLCC 10.2 1150 6.0 — — 1250 — — 930 15 24-2 XBLCC 17.8 1100 — 800 90 1100 — — 930 15 25 AA BLCC 15.9 1230 8.8 — —1250 — — 920 15 26 BB BLCC 14.7 1230 8.6 — — 1250 — — 920 15 27 CC BLCC13.7 1230 7.2 — — 1250 — — 920 15 28 DD BLCC 17.7 1230 8.5 — — 1250 — —920 15 29 EE BLCC 15.3 1230 7.3 — — 1250 — — 920 15 30 FF BLCC 14.1 12309.2 — — 1250 — — 920 15 Toughness Quenching after Fracture pipe makingAppearance Temp. Strength Transition Thickness Cooling after TemperingYield Temp. of Rate cooling Temp. Time Strength (FATT) pipe wall TestNo. (° C./sec) (° C.) (° C.) (° C.) (MPa) (° C.) (mm) Note (*) 15-1 8.250 590 20 746 −32 35 I 15-2 15.7 37 590 20 769 4 35 C 16-1 13.6 66 56020 572 −49 33 I 16-2 17.9 166 630 20 596 3 33 C 17-1 9.5 39 580 10 585−41 40 I 17-2 14.4 54 740 10 460 −21 40 C 18-1 19.9 30 630 20 538 −31 30I 18-2 17.5 64 610 20 556 −11 30 C 19-1 16.4 68 640 10 739 −27 35 I 19-210.8 40 620 30 712 −4 35 C 20-1 9.1 53 620 20 646 −53 50 I 20-2 3.1 30630 30 597 −17 50 C 21-1 19.9 53 640 10 711 −41 30 I 21-2 8.2 51 610 10733 −11 30 C 22-1 14.3 29 610 30 878 −27 32 I 22-2 18.3 64 570 20 882 532 C 23-1 13 58 640 30 876 −22 40 I 23-2 8 203 640 30 887 16 40 C 24-19.3 71 560 20 900 −27 45 I 24-2 4.8 30 620 20 845 4 45 C 25 14.4 40 60030 848 −15 35 C 26 16 38 600 30 846 −8 35 C 27 13.3 32 600 30 855 −16 40C 28 13.3 32 600 30 884 −14 40 C 29 14.5 36 600 30 901 −9 35 C 30 15.933 600 30 849 −6 35 C(*) Note: The symbol I shows the invention and the symbol C shows thecomparative.

INDUSTRIAL APPLICABILITY

According to the present invention, by restricting the chemicalcomposition of a seamless steel pipe and the manufacturing methodthereof, a seamless steel pipe, even a heavy wall steel pipe, for linepipe excellent in toughness while having high strength such as yieldstrength of X70 class (yield strength of not less than 482 MPa), X80class (yield strength of not less than 551 MPa), X90 class (yieldstrength of not less than 620 MPa), X100 class (yield strength of notless than 689 MPa), and X120 class (yield strength of not less than 827MPa) can be manufactured. The seamless steel pipe of the presentinvention is a steel pipe that can be laid in a severer circumstance ofthe deep sea, particularly, for the use of a submarine flow line. Thepresent invention is thus greatly contributable to stable supply ofenergies.

1. A heavy wall seamless steel pipe for line pipe with a high strengthand increased toughness, which has a chemical composition, by mass %,that consists of C: 0.03 to 0.08%, Si: not more than 0.25%, Mn: 0.3 to2.5%, Al: 0.001 to 0.10%, Cr: 0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02to 1.2%, fi: 0.004 to 0.010%, N: 0.002 to 0.008%, and 0.0002 to 0.005%,in total, of at least one selected from Ca, Mg and REM, and the balanceFe and impurities, optionally including V: 0 to 0.08%, Nb: 0 to 0.05% orCu: 0 to 1.0%, and that P and S among impurities are not more than 0.05%and not more than 0.005% respectively.
 2. A heavy wall seamless steelpipe for line pipe with a high strength and increased toughness, whichhas a chemical composition, by mass %, that consists of C: 0.03 to0.08%, Si: not more than 0.25%, Mn: 0.3 to 2.5%, Al: 0.001 to 0.10%, Cr:0.02 to 1.0%, Ni: 0.02 to 1.0%, Mo: 0.02 to 1.2%, Ti: 0.004 to 0.010%,N: 0.002 to 0.008%, B: 0.0003 to 0.01%, and 0.0002 to 0.005%, in total,of at least one selected from Ca, Mg and REM, and the balance Fe andimpurities, optionally including V: 0 to 0.08%, Nb: 0 to 0.05% or Cu: 0to 1.0%, and that P and S among impurities are not more than 0.05% andnot more than 0.005% respectively.
 3. A method of manufacturing a heavywall seamless steel pipe for line pipe with a high strength andincreased toughness characterized by comprising the following steps (a)to (e): (a) Forming a billet with a round cross section by continuouscasting of molten steel that has the chemical composition according toclaim 1 or
 2. (b) Cooling the billet to the room temperature at not lessthan 6° C./min of an average cooling rate between 1400 and 1000° C. (c)Heating the billet to the temperature between 1150 and 1280° C. at notmore than 15° C./min of an average heating rate between 550 and 900° C.,then piercing and rolling, to make a seamless pipe. (d) Cooling forcedlythe seamless pipe to a temperature of not higher than 100° C. at notless than 8° C./min of an average cooling rate between 800 and 500° C.,immediately after pipe making, or after isothermal treating at thetemperature between 850 and 1000° C. immediately in succession with pipemaking, or after heating to the temperature between 850 and 1000° C.after once cooling in succession with pipe making. (e) Tempering theseamless pipe at the temperature between 500 and 690° C.
 4. A method ofmanufacturing a heavy wall seamless steel pipe for line pipe with a highstrength and increased toughness characterized by comprising thefollowing steps (a) to (0): (a) Forming a bloom or slab with a squarecross section by continuous casting of molten steel that has thechemical composition according to claim 1 or
 2. (b) Cooling the bloom orslab to the room temperature at not less than 8° C./min of an averagecooling rate between 1400 and 1000° C. (c) Heating the bloom or slab tothe temperature between 1150 and 1280° C. at not more than 15° C./min ofan average heating rate between 550 and 900° C., and then cooling to theroom temperature, to form a billet with a round cross section by forgingand/or rolling. (d) Heating the billet to the temperature between 1150and 1280° C., then piercing and rolling, to make a seamless pipe. (e)Cooling forcedly the seamless pipe to a temperature of not higher than100° C. at not less than 8° C./min of an average cooling rate between800 and 500° C., immediately after pipe making, or after isothermaltreating at the temperature between 850 and 1000° C. immediately insuccession with pipe making, or after heating to the temperature between850 and 1000° C. after once cooling in succession with pipe making. (f)Tempering the seamless pipe at the temperature between 500 and 690° C.5. A method of manufacturing a heavy wall seamless steel pipe for linepipe with a high strength and increased toughness characterized bycomprising the following steps (a) to (e): (a) Forming a billet with around cross section by continuous casting of a molten steel that has thechemical composition according to claim 1 or
 2. (b) Cooling the billetto the room temperature at not less than 6° C./min of an average coolingrate between 1400 and 1000° C. (c) Isothermal treating the billet duringnot less than 15 minutes at the temperature between 550 and 1000° C.,and heating to the temperature between 1150 and 1280° C., then piercingand rolling, to make a seamless pipe. (d) Cooling forcedly the seamlesspipe to a temperature of not higher than 100° C. at not less than 8°C./min of an average heating rate between 800 and 500° C., immediatelyafter pipe making, or after isothermal treating at the temperaturebetween 850 and 1000° C. immediately in succession with pipe making, orafter heating to the temperature between 850 and 1000° C. after oncecooling in succession with pipe making. (e) Tempering the seamless pipeat the temperature between 500 and 690° C.
 6. A method of manufacturinga heavy wall seamless steel pipe for line pipe with a high strength andincreased toughness characterized by comprising the following steps (a)to (0): (a) Forming a bloom or slab with a square cross section bycontinuous casting of a molten steel that has the chemical compositionaccording to claim 1 or
 2. (b) Cooling the bloom or slab to the roomtemperature at not less than 8° C./min of an average cooling ratebetween 1400 and 1000° C. (c) Isothermal treating the bloom or slabduring not less than 15 minutes at the temperature between 550 and 1000°C., and heating to the temperature between 1150 and 1280° C., thenforging and/or rolling, to form a billet with a round cross section, andthen cooling to the room temperature. (d) Heating the billet to thetemperature of 1150 to 1280° C., then piercing and rolling, to make aseamless pipe. (e) Cooling forcedly the seamless pipe to a temperatureof not higher than 100° C. at not less than 8° C./min of an averagecooling rate between 800 and 500° C., immediately after pipe making, orafter isothermal treating at the temperature between 850 and 1000° C.immediately in succession with pipe making, or after heating to thetemperature between 850 and 1000° C. after once cooling in successionwith pipe making. (f) Tempering the seamless pipe at the temperaturebetween 500 and 690° C.